1. Field of the Invention
This invention relates generally to a system and method for perforating a web or film with a beam of high intensity, coherent radiation and for controlling the resultant porosity of the web to provide a precisely-controlled, uniform porosity of the finished product.
2. Description of the Prior Art
The prior art has described as in U.S. Pat. No. 4,025,752 by the inventor of this invention and assigned to the assignee of this invention, a system for controlling the porosity of a web or film by controlling the frequency or pulse width of an electric spark passing between electrodes and through the web. The web is driven past the pair of electrodes and the resultant porosity is measured to be compared with a reference set point to develop an error signal. The resultant error signal is multiplied with a signal indicative of web speed to control the frequency or pulse width of a pulse generator, which is connected to cause arcing between the aforementioned electrodes.
Further, the prior art discloses the use of a high intensity, coherent beam of radiation such as produced by a laser to perform various machining functions including cutting or placing holes in materials including webs of materials directed therepast. For example, U.S. Pat. No. 4,063,064 of Saunders et al. discloses apparatus for precisely forming holes by laser beams within work pieces such as pills. The work piece is moved past the laser beam directed onto the work piece, and the size of the hole is controlled by controlling the focus of the laser beam, and in particular by controlling the distance between a focusing lens and the work piece, whereby the diameter of the beam and therefore the resultant hole may be controlled.
In U.S. Pat. No. 4,032,743 of Erbach et al., there is disclosed a method and apparatus for forming rows and columns of closely spaced holes within a thin foil, whereby the resultant porosity in terms of percentage of openness may be controlled. In particular, the thin film is rotated by a drum past a series of lasers whose focal distance from the foil is controlled by means of micrometer threads to thereby control the size of the openings within the foil. The frequency of the laser firing is controlled by applying a track of magnetic data upon an end portion of the drum that is sensed magnetically to provide control signals to turn on and off the lasers, whereby the center-to-center spacing and the row spacing of the resultant holes is controlled. In a further embodiment, a rotating disk is used to interrupt a light source whereby control signals are developed to control the laser firing and therefore the center-to-center and row spacing. However, there is no means for directly sensing or measuring the porosity, and the percentage of openness of the film is controlled by determining the size of the opening, as well as the center-to-center spacing of the openings and the spacing between rows. Once the focus and therefore the size of the opening formed by the laser beam is set, it is no longer adjusted.
Further, in U.S. Pat. No. 3,226,527 of Harding, there is disclosed apparatus for perforating a sheet material that is directed past an array of lasers. In a further embodiment, there is suggested that a single laser be scanned across the lateral dimension of the web as it is directed past the scanning laser. The spacing between the perforations is controlled by a rotating mechanical shutter that is rotated in synchronism with the speed at which the web is directed past the laser beam. The mechanical shutter is rotated by a motor and has openings to permit the passage of the laser beam onto the web to be perforated. By controlling the relative rates of interruption of the laser beam and the speed at which it is directed therepast, the spacing between perforations is controlled.
It is noted that the above-described apparatus and methods of the prior art does not directly measure porosity but rather controls porosity by controlling the size of the perforations as well as the spacing therebetween. Thus, the prior art assumed that hole density and thus porosity could be controlled by varying the laser pulse frequency relative to web speed. However, since the continuous laser beam is multiplexed, i.e, is shuttered on and off as suggested by U.S. Pat. No. 3,226,527, it has been discovered by this invention that the ratio of perforated area to unperforated area remains unchanged regardless of the pulse rate and/or web speed. This is may be understood from the following observations. If for example, the rate of perforation is increased by increasing the speed of the mechanical shutter, there will be no substantial change in the resultant web porosity. First consider if the frequency is increased, the length of time that a portion of the web is exposed to the laser beam is decreased and therefore the resultant size of the hole is decreased. Thus, though the frequency or number of holes increases, the resultant size decreases thereby tending to maintain the resultant porosity of the web constant. Conversely, if the speed of rotation of the mechanical shutter is decreased, the length of exposure to the beam of radiation and thus the resultant size of the perforation is increased. Though the size of the opening is increased, the number of such openings or perforations is decreased thus tending to maintain the resultant porosity constant. However, there will be a slight change in web porosity at moderately high and low web speeds and pulse rates. This is due to the limited burn duration of each pulse applied to the moving web which, itself, exhibits varying degrees of thermal inertia.